Which medieval narrative songs are you referring to, Jack? All the work I'm doing is on narrative (which is indeed long, but not particularly repetitive) and there is no evidence at all as to how it would have been performed (as in: as a song or as a story or a mixture of both). It is in octosyllabic rhyming couplets and may well have had some parts accompanied by musicians, but there are absolutely no clues as to which parts or how. Believe me, I've looked! So I'm puzzled about your reference to medieval texts presented as songs - I'm not convinced they were, but maybe you have something specific in mind?
Length of performance is something else I've been considering, as it is most relevant to my studies, and again there is nothing definitive anywhere. It doesn't help, of course, that the notion of, say, 20 minutes, is a far more modern construct. But in the work I'm studying there are a number of discrete episodes of varying lengths as well as some interconnected ones which are far more difficult to disentangle. If you talk to Shonaleigh, working in a storytelling tradition, you'll find that audiences would have listened for hours on end to stories, but I haven't found any historic references to that taking place in the time and context I'm looking at.

The other aspect that this touches on is attention span. There is no way to ascertain what this might have been, or whether it was different in the past, as even today there is a huge variance among studies as to what an "average" attention span might be. My own experience working with small children (notoriously difficult to engage for long) is that with stories that grab their interest there is no problem keeping them still for up to 45 minutes, but the key is grabbing their interest.

I'm currently looking for willing volunteers to hear "my" story from start to finish, because so far, with the constraints of storytelling and folk clubs, the longest session I've managed to tell for is an hour and a half (plus a comfort break). I think a colloquial English telling, not in octosyllabic couplets and avoiding some of the digressions onto the nature of love but including all the plot lines, would probably take 3 - 4 hours (plus breaks). I don't think, however, that back in the thirteenth century they would have told it all in one go. But I could be wrong.

However, I think there is a huge difference between a live performance and a recorded one, so "putting the thing on record" is a sure-fire way, I would think, of killing it stone dead, unless it was a record of an experience for the audience present at the time. Again, I'm looking at what other stimuli (visual, auditory) might assist in a long telling of "my" story, but talking to other storytellers there is no consensus that this would necessarily help a great deal.